This disclosure relates to reducing distortion in magnetic resonance images.
Magnetic resonance imaging (MRI) has been widely applied in medical, biological, and other fields. Typically, MRI produces an image of a part of an object under examination by manipulating the magnetic spins and processing measured responses from the magnetic spins. An MRI system can include hardware to generate different magnetic fields for imaging, including a static magnetic field along a z-direction to polarize the magnetic spins and gradient fields along mutually orthogonal x, y, or z directions to spatially select a body part for imaging, as well as hardware to generate a radiofrequency (RF) field to manipulate the spins. An MRI system can also include a data processing device for filtering, displaying, and otherwise manipulating the generated data.
Actual magnetic fields in real-world MRI systems, including the static magnetic field (B0) and gradient fields, deviate from ideal. Such deviations can cause distortions in the acquired MRI images. For example, a static magnetic field (B0) may be spatially inhomogeneous and gradient fields may have nonlinearities in their gradients. The distortions caused by such imperfections can limit the accuracy and usefulness in detecting certain conditions in brain and other tissue. For example, MRI images acquired by the high-speed Echo Planar Imaging (EPI) can suffer sever distortions caused by imperfections in applied fields. Hence, it is desirable to reduce such field-induced distortions.
In an article entitled “A Technique for Accurate Magnetic Resonance Imaging in the Presence of Field Inhomogeneities,” (IEEE Trans. Med. Imaging, Vol. 11, p. 319-329 (1992), the contents of which are incorporated herein by reference), Chang and Fitzpatrick assert that the correct, undistorted MRI image lies midway between two MRI images produced using magnetic fields with opposite gradients of the same magnitude.
Because of its ultra-fast imaging speed and relatively good image definition, Blipped Echo Planar Imaging (EPI) is the pulse sequence of choice for diffusion and perfusion imaging, functional MRI (fMRI), cardiac imaging, and abdominal imaging. However, because inhomogeneous static magnetic field-induced distortions in, for example, a 3T scanner can easily be a centimeter or more near an air-tissue boundary such as the sinuses, the utility of EPI is reduced.
Morgan et al. have proposed a technique to reduce such magnetic field distortions in Blipped Echo Planar Imaging (EPI). See “Correction of Spatial Distortion in EPI Due to Inhomogenous Static Magnetic Fields Using the Reversed Gradient Method,” J. Magn. Reson. Imaging Vol. 19, p. 499-507 (2004), the contents of which are incorporated herein by reference. It is believed that this technique does not accurately reflect continuous and smooth image distortions caused by generally continuous and smooth magnetic field distortions, as discussed further below.